Field of the Invention
The disclosure relates generally to a DC-DC switching power converter, employing a pulse-width modulation (PWM) control signal and different modes of operation.
Description of Related Art
The Buck-Boost switching converter is a type of DC-DC switching converter that has an output voltage magnitude that is either greater than or less than the input voltage magnitude. It is a switch mode power supply with a similar circuit topology to the Buck switching converter and the Boost switching converter. The output voltage is adjustable based on the duty cycle of the switching transistor.
FIG. 1a shows a prior art Buck-Boost switching configuration having 1 inductor and 4 switches, of U.S. application Ser. No. 13/847,574, owned by a common assignee, and herein incorporated by reference in its entirety. Switches S1-S4 are usually realized by transistors. While in the on-state, S1 and S3 are closed (on) and the input voltage source is connected to the inductor L. This results in accumulating energy in L. In this stage, the capacitor Cout supplies energy to the output load. While in the off-state, S2 and S4 are closed (on) and the inductor L is connected to the output load and capacitor Cout, so energy is transferred from L to Cout and Load. The characteristics of the Buck-Boost switching converter are mainly that the output voltage can vary continuously from 0 to ∞ (for an ideal converter), compared to the output voltage varying from 0 to Vin and Vin to ∞, for a Buck and a Boost switching converter.
The classical Buck-Boost switching control uses only one PWM control signal and employs only one mode of operation. It suffers poor conversion efficiency due to its high switch driving loss and high inductor current requirement.
One example of the prior art, U.S. Pat. No. 6,166,527 (Dwelley et. al), employs three different modes of operation, which are Buck, Buck-Boost, and Boost switching modes, by using more than one PWM control signal to improve the efficiency.
Another example is U.S. Pat. No. 8,860,387 (Kobayashi), also owned by a common assignee, and herein incorporated by reference in its entirety, employs five modes of operation, which are Buck, half-frequency (fsw/2) Buck, fsw/2 Buck-Boost, fsw/2 Boost, and Boost switching modes. This system has the ability to reduce the switching frequency in half and improve the efficiency when the input voltage is close to the output voltage while maintaining regulation performance.
These Buck-Boost switching converters using different operation modes might have a mode bounce issue, which occurs at the transition of two switching modes. If mode bounce occurs, the converter goes back and forth between two operation modes and is unstable. As a result, the output voltage suffers from significant ripples and the efficiency is degraded.
FIGS. 1b & 1c illustrate relationship 100 between input voltage and error amplifier output, for constant output voltage and different modes of operation of a Buck-Boost DC-DC switching converter, of the prior art. Buck, Buck-Boost, and Boost switching modes are illustrated in the waveform of FIG. 1b and Buck, half-frequency (fsw/2) Buck, fsw/2 Buck-Boost, fsw/2 Boost, and Boost switching modes are illustrated in the waveform of FIG. 1c. What is shown is that one way to avoid mode bounce is to add an offset voltage on the error amplifier output when the operation mode changes. This offset voltage generates hysteresis of the input voltage threshold for mode transitions. Larger offset voltage decreases mode bounce likelihood, but causes a larger output disturbance during the transition.
FIG. 2 illustrates behavior 200 of output voltage and error amplifier output, at a mode transition of a Buck-Boost DC-DC switching converter, of the prior art. When the input voltage crosses a threshold, mode transition occurs. Error amplifier output 210 is shown with a given offset voltage, causing undershoot during mode transition on output voltage 220.
Another example of the prior art employs a hysteresis at the error amplifier output, as disclosed in U.S. application Ser. No. 13/847,574. FIG. 3 shows relationship 300 between input voltage and error amplifier output, with hysteresis of the error amplifier output during mode transition of a Buck-Boost DC-DC switching converter. Increasing the error amplifier output voltage range for the present operating mode of Buck, Buck-Boost or Boost generates hysteresis. However some offset voltage on the error amplifier output is still needed, when the mode transition occurs, resulting in some overshoot and undershoots until the error amplifier output settles at the proper voltage level. If a mode transition occurs during load and/or line transient, the overshoot and undershoots become larger.